The invention relates to implantable biomedical interfaces, and more particularly to a cuff for biological soft tissue which can be used as an electrode for selective stimulation and/or monitoring of nerve groups. Likewise, the cuff can be used as a delivery system for localized application of medication, such as bachlofin. The cuff also has application for use as a sensor for chemical removal.
As the level of sophistication has increased in biomedical arts, advances have been made in implantable therapies. Therapies have evolved which involve the precise application of stimulus including electrical stimulus medication.
This invention relates in particular to a nerve electrode cuff for use in functional electrical stimulation. Functional electrical stimulation of the nervous system can be used to help to restore or maintain some degree of lost sensory and motor function in neurologically impaired individuals. In addition, there are certain specialized applications, such as the treatment of sleep apnea, where it is necessary to simultaneously monitor and generate electrical signals in nerves. The current invention extends to a method which both monitors and generates electrical signals in nerves, in particular of the hypo-glossal nerve for treatment of sleep apnea.
Prior art methods and apparatus which can be used in functional electrical stimulation and/or recording to restore a particular function broadly include:
(1) surface electrodes placed on the skin surface to activate nerves in a general region of interest;
(2) intra muscular and epimysial electrodes to activate nerves to individual muscles; and
(3) the use of neural interfaces to address individual nerves.
Specific research in the area of nerve interfaces has involved nerve cuffs for stimulating and monitoring nerve activity. Cuff electrodes are used in peripheral nerve stimulation and produce function with up to 1000 times less charge than required by either surface or intra muscular electrodes. For example, peripheral nerve stimulation with a cuff requires less than 100 nC for full functional recruitment compared to up to 10 xcexcC with surface stimulation or up to 4 xcexcC for intramuscular electrical stimulation. The smaller power requirement may result in a potentially safer long term therapy. Other advantages of peripheral nerve electrodes include the fact that an entire muscle can be recruited from a single electrode. In addition, it may be easier to stimulate an appropriate peripheral nerve rather than some muscles which are difficult or impractical to implant with IM or epimysial electrodes (for example as in the larynx).
Prior art cuff electrodes have included proximity electrodes which are sutured into position. These electrodes require a relatively high amount of current. Half cuff electrodes are generally C-shaped, while cylindrical electrodes can be spiral, helical, split-cylinder, or chambered cylinders. C-shaped or split cylinder electrodes generally include a cylinder of dielectric material finding a bore having sufficient diameter to receive a nerve trunk to be electrically stimulated. Single or multiple annular electrodes can be positioned on the inner surface of the bore for applying electrical stimuli. The electrical stimuli, for example, may be used to provide functional electrical stimulation, to block neural nerve impulses traveling along the nerve trunk, or to cause other effects.
The spiral type of cuff electrode typically includes a self-curling sheet of non-conductive material biased-curl into a spiral. Conductive strips or pads are disposed on the self-curling sheet extending peripherally around the inner surface of the cuff. The conductive segments may be electrically conductive for applying electrical impulses or fluid conductive for infusing or extracting medications. In use, a first edge of a self-curling sheet may be disposed adjacent a nerve truck around which the cuff is positioned. The self-curling sheet is permitted to curl around the nerve forming an annular cuff. Helical electrodes wind around the nerve like a spring allowing nerve flex and fluid exchange with surrounding media (i.e. tissue).
Another approach to electrical stimulation of the nervous system involves small wire electrodes which penetrate the perineurium membrane and are advanced into a fascicle of the nerve, within fascicular endoneurium. This method has the disadvantage of being highly invasive and can result in permanent damage to the nerve through penetration of the perineurium and mechanical trauma to the axons.
Regeneration type neural interfaces are comprised of a thin silicon diaphragm with many small holes, which is positioned between the cut ends of a peripheral nerve. Over time the axons will regenerate through the many small holes in the diaphragm. A disadvantage to this therapy is that it requires the nerve to be severed. As well, axons tend to regenerate around the interface rather than through it.
The present invention contemplates a soft tissue cuff for use for example, as a nerve cuff electrode. The invention has application in addition for medicinal infusers and implantable biomedical devices for introducing, monitoring, or removing matter, fluids or energy. In contrast to prior art cuffs, the present invention is intended to apply a small, non-circumferiential force over time; this results in a non-damaging pressure within the intrafascicular endonurium so as to effect the nerve shape but not as to occlude blood flow within the nerve. The cuffs of the present invention may be implanted without damage to the subject nerve. Further, the cuff can allow for tissue swelling and movement. The present invention causes the nerve to mimic its natural reaction to forces applied within the body. Specifically, the body naturally applies small forces to the nerves which results in flattening or other shape changes to the nerve. An example of these forces is illustrated by the flattening of the sciatic nerve as it exits the pelvis at the sciatic foramen. Further, some nerves will take on an ellipsoidal shape as they pass through muscle planes. Other nerves have demonstrated significant flattening over time as a result of tumor pressure.
In a preferred embodiment, the present invention recognizes a small range of pressure which will cause nerve reshaping without damaging the nerve. In particular, this cuff electrode is designed to apply a force that does not cause pressure to rise above 40 mmHG (or more specifically, does not cause a reduction of blood flow to less than 70% of normal (i.e., baseline)). It is currently believed that tissue pressure correlates with blood flow. Specifically, at less than 10 to 15 mmHG, there is little effect on blood flow. At the range of 15 to 20 mmHG the venus blood flow is initially impaired. At 30 mmHG the capillary and arterial blood flow is first impaired. By 80 mmHG, neural blood flow stops completely. In compressive pathologies, such as carpal tunnel syndrome, damage and pain do not occur until the neural pressure is greater than 30 mmHG. Consequently, the current invention is intended to apply a force resulting in an internal nerve pressure of between about 5 mmHG and 40 mmHG, and more preferably between 15 mmHG and 30 mmHG, and even more preferably from 15 mmHG to 20 mmHG.
The current invention has the object of solving the deficiencies in the prior art. The present invention has the advantage of providing selectively, i.e. the ability to activate and record a specific population or subset of axons within a nerve.
A soft tissue cuff is provided which is non-invasive to the soft tissue. Further, the electrode is more compact in at least one dimension and may lower charge requirements as it allows for more selective stimulate on specific axons, and especially the central axons. Since the perineurium is reshaped over time, damage to this protective neural tissue is minimized.
In accordance with the present invention a nerve cuff is provided which applies the defined pressure to a nerve to cause gradual reshaping over time. This pressure is applied in a way to allow the nerve to adapt to the condition without damage to the nerve. Of course, it should be understood that the amount of time will vary with the nerve as some nerves may adapt faster than others. The required time for reshaping may be 24 hours or as little as 2 hours or up to 1 week or more. The quality and condition of the nerve will also contribute to this time for reshaping.
In a first embodiment, the invention includes an elongated, substantially rectangular central opening having a height, which is smaller than and a width which is longer than the diameter of the nerve to which it is applied. The cuff may be made of a material having a sufficient elasticity and a shape sufficient to cause a force applied selectively across the transverse direction of the nerve. The nerve cuff can be open at a single end or alternatively the nerve cuff may be open at two ends. The open ends are closed such as by staple, an O-ring in a grooved area, a suture, a mechanical interference fit or other closure mechanism. The beams which form the top and/or bottom of the nerve cuff and the connecting juncture for these beams have a structure and/or material characteristic tailored to impart a particular pressure to the nerve.
In a further embodiment, the invention can be used both to record sensory neural activity and stimulate motor output. This capability from a single device would be very beneficial for closed loop systems in applications such as restoring hand grasp or obstructive sleep apnea.